1. Field of the Invention
The invention relates to a process for the preparation of a polymer alloy having multiphase morphology, that includes intimately mixing thermoplastically processable polymers, which are incompatible or not very compatible, in the melt. The invention furthermore relates to a molding produced by this process and the use of such moldings.
2. Description of Related Art
The modification of polymeric materials in an injection molding machine by addition of additives such as dyestuffs or stabilizers, either directly or in masterbatch systems, is known.
U.S. Pat. No. 5,130,076 describes a process for distributing small amounts of an incompatible thermoplastic in finely dispersed form in a thermoplastic matrix that is the main constituent of a polymer blend, by introduction of shear energy such that the incompatibility of the plastics does not lead to a reduction in the mechanical properties. This process is particularly suitable for modification of elastomers, for example, recycled plastic. However, relatively high concentrations of the second blend partner cannot be achieved, since the incompatibility of the components would lead to a decrease in the mechanical properties.
Furthermore, at a high concentration of the incompatible blend polymer, agglomeration and thus phase separation are to be expected during the mold-filling phase, regardless of the mechanical shear energy introduced. This leads to a laminar phase structure. Furthermore, the phase separation causes non-uniform material properties in the molding. This process therefore allows only the preparation of polymer alloys comprising a matrix component and not more than 25% of an incompatible modifying component. Many alloys of industrial interest are therefore not accessible by this process.
DE 44 43 153 describes a process for the preparation of polymer blends in which the components are initially introduced into the apparatus as a powder mixture, and part of the mixing function of the actual processing by injection molding is thus carried out beforehand. The advantage of this process lies in the lower exposure to shear and therefore lesser damage to the components. This effect is particularly advantageous in the production of light-colored moldings. However, this process proves to be expensive if the components are present not as a powder but as granules after the polymerization. Such plastics must first be ground, during which thermo-mechanical damage may occur. To reduce this damage, for example, the plastics can be cooled with liquid nitrogen, but this increases the costs for preparation of the powder.
The difficult flow properties of mixtures of plastics powders in the hopper of the injection molding machine also proves to be a disadvantage. For example, if the two components have different particle sizes, demixing may occur, the consequence of which is non-uniform properties in the moldings. A stirrer in the hopper of the injection molding machine prevents demixing effects and also bridging. However, due to the friction of the particles of the plastics against one another caused by the stirrer, electrical charging may occur, which differs in degree in the various components of the plastics, depending on the electrical conductivity, and can thus lead to different feed properties. Furthermore, such a stirrer increases the overall costs of the process.
Moreover, cleaning work during storage and transportation of powders of plastics, for example when changing materials, are expensive.